The present invention relates to a process for producing shape strips of metals that are suitable for use as materials for high-grade terminal leads and lead frames, etc.
The term "shape strips" herein used means a long piece of solid material whose longitudinal dimension is much larger than its cross-sectional dimension having different thicknesses in the direction of width, and includes bars, rods, sheets, plates or the like having profiled cross section whose thickness varies in the direction of width. One of the representative examples of such shape strips is shown by its cross-section in FIG. 2 with the reference number 7.
Prior art techniques that have been proposed and practiced for producing shape strips of metals include the method of using a V-shaped die and flat surfaced rollers and the method of rolling by means of the combined use of a grooved roll and a flat roll. The first method is described in Japanese Patent Publication No. 908/1977 and comprises the steps of placing a flat workpiece in a receiving mold having a groove in the inner surface and deforming the flat workpiece in the direction of width by rotating flat rollers installed on the opposite side of said receiving mold in a direction that crosses the longitudinal direction of the workpiece. The second method is described in Japanese Patent Publication No. 30563/1989 and is characterized by using work rolls at least one of which is grooved. In this method, a workpiece is rolled with the rolling force being applied only to the area where the thickness of the workpiece should be reduced in the direction of width so that the part of the workpiece positioned in the groove in the grooved roll will deform by buckling, and the buckling part is subsequently flattened with the flat roll.
In these and other prior art methods, a flat workpiece is worked into various shapes, so the reduction ratio varies in areas having different thicknesses and there is a working limit as regards the thickness ratio between thin and thick portions of the strip. Because of these problems, the prior art methods have had the following defects:
1) the working operations are cumbersome and inevitably require large equipment; PA1 2) because of the difference in thickness reduction ratio (draft) in areas having different thicknesses, structural differences tend to remain even after annealing and such structural differences will cause residual strain; PA1 3) it is difficult to produce shape strips of a complex profile such as one having two or more ridges; and PA1 4) high production rate is difficult to achieve.
The present invention has been achieved under these circumstances aiming at developing a new process by which a shape strip of high quality that is finished to have a desired cross sectional profile can be produced without causing any substantial working deformation in the workpiece.